Olefin polymerization catalyst systems may be divided into Ziegler-Natta and metallocene catalysts, and these highly active catalyst systems have been developed in accordance with their characteristics. Ziegler-Natta catalyst has been widely applied to commercial processes since it was developed in the 1950's. However, since the Ziegler-Natta catalyst is a multi-active site catalyst in which a plurality of active sites are mixed, it has a feature that a resulting polymer has a broad molecular weight distribution. Also, since a compositional distribution of comonomers is not uniform, there is a problem that it is difficult to obtain desired physical properties.
Meanwhile, the metallocene catalyst includes a main catalyst having a transition metal compound as a main component and an organometallic compound cocatalyst having aluminum as a main component. Such a catalyst is a single-site catalyst which is a homogeneous complex catalyst, and offers a polymer having a narrow molecular weight distribution and a uniform compositional distribution of comonomers, due to the single site characteristics. The stereoregularity, copolymerization characteristics, molecular weight, crystallinity, etc. of the resulting polymer may be controlled by changing a ligand structure of the catalyst and polymerization conditions.
U.S. Pat. No. 5,914,289 discloses a method for controlling a molecular weight and a molecular weight distribution of polymers using metallocene catalysts which are respectively supported on supports. However, a large amount of solvent and a long period of time are required to prepare the supported catalysts, and a process of supporting metallocene catalysts on the respective supports is troublesome.
Korean Patent Application No. 2003-12308 discloses a method for controlling molecular weight distributions of polymers, in which the polymerization is performed while changing a combination of catalysts in a reactor by supporting a dinuclear metallocene catalyst and a mononuclear metallocene catalyst on a support together with an activating agent. However, this method has limitations in simultaneously realizing the characteristics of respective catalysts. In addition, there is a disadvantage that the metallocene catalysts are departed from a supported component of the resulting catalyst to cause fouling in the reactor.
Therefore, to solve the above drawbacks, there is a continuous demand for a method for preparing polyolefins with desired physical properties by easily preparing a supported metallocene catalyst having an excellent activity.
Meanwhile, a linear low density polyethylene (LLDPE) is produced by copolymerizing ethylene and an alpha olefin using a polymerization catalyst under low pressure. Thus, this is a resin having a narrow molecular weight distribution and a certain length of a short chain branch (SCB), without long chain branch (LCB). LLDPE film has high strength at break and elongation in addition to the properties of a general polyethylene and exhibits excellent tear strength, falling weight impact strength or the like. This has led to an increase in the use of a stretch film, an overlapping film or the like which is difficult to apply existing low density polyethylene or high density polyethylene.
A process of preparing linear low density polyethylene using 1-butene or 1-hexene as a comonomer is generally performed in a single gas phase reactor or a single loop slurry reactor, and its productivity is higher than a process using 1-octene comonomers. However, due to limitations of catalyst and process technologies, the product has physical properties inferior to those of a product obtained by using 1-octene comonomers, and has a narrow molecular weight distribution to show poor processability. Many efforts have been made to improve these problems.
U.S. Pat. No. 4,935,474 discloses a method of preparing polyethylene with a broad molecular weight distribution by using two or more metallocene compounds. U.S. Pat. No. 6,828,394 discloses a method of preparing polyethylene having excellent processability, in particular, being suitable for films by using a mixture of a good comonomer incorporator and a poor comonomer incorporator. In addition, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose that polyethylene having a bimodal or multimodal molecular weight distribution is prepared by using a metallocene catalyst containing at least two metal compounds, thereby being applied to films, blow molded products, pipes, and the like. However, even though these products have improved processability, the dispersion state according to the molecular weight in a unit particle is not uniform, and thus appearance is rough and physical properties are not stable under relatively mild extrusion conditions.
Under this background, there is a continuous demand for preparation of an excellent product in which a balance between physical properties and processability is ensured, and especially a polyethylene copolymer having excellent processability is further required.